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Magnetostrictive Sensing Ensures Accurate Level Monitoring

Sunday, 01 February 2009

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For process industries, one of the
greatest challenges plant operators
face is finding a sensor technology that
will provide reliable measurement of liquid
levels in tanks while requiring minimal
attention and maintenance.

Magnetostrictive sensing technology is
used across a broad range of industries,
including pharmaceutical, food and beverage,
chemical and petrochemical, to
monitor liquid levels. There has been a
large acceptance of magnetostrictive
level transmitters by the process industries
due to the ease of use, simple design
and years of reliable performance.

The general application of a level
transmitter can be simplified to a vessel
or tank, a level transmitter, and a host.
The vessel has a certain amount of liquid
to that needs to be monitored to keep
the process working. If there is not
enough liquid, the process stops and if
there is too much liquid, the vessel can
overfill and spill. A level transmitter is
added to measure the liquid level and
transmit the value(s). The values can be
displayed locally for the plant operator
to view and/or tied into a host system
that automatically controls the liquid
level based on the transmitted data.

Magnetostrictive level transmitters use
the time-based magnetostrictive position
sensing principle. The level transmitter
comprises five parts: a housing, a rigid
pipe or flexible hose, electronics, sensing
element, and a float. The housing,
rigid pipe or flexible hose, and float create
a package around the sensing element
and electronics protecting them
from the process conditions. Within the
sensing element, a sonic strain pulse is
induced in a specially designed waveguide
by the momentary interaction of
two magnetic fields. One field is generated
by a permanent magnet sealed
inside the float, while the other is generated
from an ’interrogation’ current
pulse applied along the waveguide from
the electronics.

The resulting strain pulse travels at
ultrasonic speed along the waveguide
and is detected at the head of the sensing
element. The magnet’s position is
determined by accurately measuring the
elapsed time between the application of
the interrogation pulse and the arrival
of the resulting strain pulse. It provides
an absolute position reading that never -
needs recalibration after a power loss.

The main feature of using magnetostrictive
level transmitters is the accuracy
they achieve. Specific magnetostrictive
level transmitters can reach accuracy
of 1/32nd in. for shorter lengths, and
1/16th inch for longer lengths of 60
feet. While this level of accuracy is
achieved by other level technologies,
magnetostrictive level transmitters do
not need recalibration and can be
installed and commissioned using hand
tools and a PC with a serial converter.
These features allow magnetostrictive
level transmitters to have the lowest
installed cost, compared to equivalent
level technologies such as radar.

One benefit of magnetostrictive level
transmitters is that the transmitter does
not need to be removed from the tank
for repairing or replacing the electronics.
A technician can easily access the
transmitter housing atop the tank and
remove the sensing element and/or electronics.
The ability to replace or repair
the electronics in the field keeps the tank
owner from having to take the tank out
of service, which reduces downtime and
lowers costs. The only reason to enter
the tank is if the pipe or float is damaged.

Another key benefit of using a magnetostrictive
level transmitter is the ability
to monitor two liquids in the same tank
from one transmitter. In some cases, the
contents of a tank may contain more
than one liquid. Magnetostrictive sensors
have the capability to use more than one
float to measure the interface of two liquids
that may be contained within a tank.

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